Wavelength division multiplexing transmission systems are remarked and widely studied as means for realizing optical fiber transmission with a large capacity over a long distance. In wavelength division multiplexing transmission systems, it is essential to make individual wavelength lights to coincide in peak values at least at a transmitter terminal. It is also preferable to make them coincide in the stage of repeating amplifiers as well.
Erbium-doped optical fibers are widely used as media for amplifying wavelength division multiplexed signal lights, but it is difficult to obtain flat amplification wavelength characteristics within wavelength bands required in wavelength division multiplexing transmission systems. FIG. 7 schematically shows optical spectrums obtained by collectively amplifying wideband wavelength division multiplexed signal lights from 1535 nm to 1560 nm, equal in power, using a single optical amplifier. As shown, peak powers of individual wavelengths are not even, and tend to exhibit an inclined or arcuate profile, depending upon characteristics of amplifying mediums (wavelength distribution characteristics of ASE light (Amplified Spontaneous Emission light) and gains). Especially, it is largely affected by wavelength distribution of ASE light in optically amplifying medium.
There have been proposed, some gain equalizing means for reducing or removing variance in peak power among individual wavelengths of wideband wavelength division multiplexed signal light, such as a structure which uses an etalon filter having transmission characteristics of canceling wavelength characteristics of an optically amplifying medium and locates it downstream of the optically amplifying medium, or a structure which uses a fluoride fiber amplifier having more flat wavelength characteristics.
It is difficult, however, to equalize gains of wavelength division multiplexed signal light of a very wide band by using etalon filters. Further, fluoride fiber amplifiers have never been used in long-distance transmission, and involve an uncertainty in reliability of fluoride fibers themselves.